Baxi 105e Lukewarm water/Rads constantly warm?

We have a Baxi 105e and recently the hotwater from a 'high demand' tap (like the shower or bath) only comes through lukewarm. I've checked the settings on the boiler and all seems fine. I have turned the cold water intake down as far as it will go without causing the boiler to cut off with seems to have improved the situation temporarily.

Meanwhile I noticed that the preheat was struggling to keep the water at a high temperature and when you run the taps the rads get warm (even when the CH is off). Again, turning Preheat off/v low seems to have improved things a little.

Any ideas? Cheers